Flexible multiconductor transmission line utilizing alternate conductors as crosstalk shields



2 Sheets-Sheet 1 SHIELD POTENTlAL SOURCE SIGNAL SOURCES SHIELD POTENTIA.I .LQ/

PAULSEN "III" SIGNAL SOURCE 27 25 PRIOR ART SIGNAL SOURCES ALTERNATECONDUCTORS AS CROSSTALK SHIELDS Filed D60. 5, 1962 FIG..'I

April 20, 1965 42 INVENTOR ROBERT c PAULSEN BY ATTORNEL SHIELD POTENTIALSOURCE April 20, 1965 R. c. PAULSEN 7 FLEXIBLE MULTICONDUCTQRTRANSMISSION LINE UTILIZING ALTERNATE CONDUCTORS AS CROSSTALK SHIELDSFiled Dec. 5 1962 2 Sheets-Sheet 2 FIG. 2

SCALE 1 INCH 25 MILS SCALE HNCH=25MILS United States Patent M FLEXBLEMULTICGNDUCTOR TRANSMlS ltlN LINE UTILIZING ALTERNATE CSNDUCTQRfi ASCROSSTALK SHIELDS Robert C. Paulsen, Poughkeepsie, N.Y., assignor tointernational Business Machines Corporation, New York,

N.Y., a corporation of New York Filed Dec. 5, 1962, Ser. No. 242,542 7Claims. (Cl. 333-1) This invention relates to transmission lines, andmore particularly, to flexible, multiple, conductor transmission linessuitable for manufacture in a continuous process.

As microminiaturization reduces the size of data processing apparatus,conventional single wire, twisted pair and coaxial cables becomeobsolete for interconnecting the various units in the apparatus. Suchinterconnecting lines are often too stiff and bulky and lack theflexibility required for bending within the small spaces ofmicrominiaturized units. Tape cables, which are flat and flexible, lendthemselves to microrniniaturized data processing apparatus. Tape cablesrequire less space as well as present a neater appearance than priorinterconnecting elements.

Flexibility and crosstalk present problems to tape cables as thecharacteristic impedance requirements of the cables increase.Conventionally, an increase in characteristic impedance requirements oftransmission lines results in a corresponding increase in the thicknessof the cable dielectric with a resultant decrease in cable flexibility.Many times, it is desirable to fold tape cables to change direction. Incertain instances, the folding brings the signal conductors in closeproximity to one another with a resultant increase in crosstalk. Also,the folding often breaks or interrupts the ground conductors of suchcables with a resultant increase in cable attenuation, characteristicimpedance as well as crosstalk. It is desirable, therefore, to improvetape cables so that flexibility, crosstalk and attenuation problems aresubstantially eliminated thereby exploiting the full potential of suchcomponents J crosstalk.

These and other objects are accomplished in accordance with the presentinvention, one illustrative embodiment of which comprises an insulatingmember having a preselected dielectric constant, a plurality of signalconductors having a diameter d and a plurality of ground conductorshaving a diameter d where d .d the signal and ground conductors beingjuxtaposed in the insulating member in a series of superimposedconductive planes. Alternate conductive planes are offset the samedirection and extent with respect to the superimposed planes so thateach signal conductor is surrounded by at least three ground conductors.The diameters of the ground and signal conductors and dielectricconstant of the insulating material cooperate to permit a thin tapecable that has little or no crosstalk when electrical signals aresupplied to the signal conductors. The cable attenuation issubstantially constant regardless of the number of folds made in thecable since the ground conductors have a strength whichwill not fracturewhen folded upon themselves.

3,l7,%4 Patented Apr. 20, 1965 One feature of the invention is aplurality of first and second conductive members positioned in thedielectric, the configuration of the conductive members being such thateach first conductive member is surrounded by at least three secondconductive members.

Another feature is a plurality of superimposed conductor planes wherebyeach conductor plane has alternate conductive members of differentdiameters, corresponding conductors in each plane being of differentdiameter.

Another feature is a tape cable that surrounds each signal path with atleast three ground conductors to nullify crosstalk among signal paths.

Another feature is a tape cable having a plurality of superimposedconductor planes wherein each plane comprises a plurality of ground andsignal conductors in juxtaposed relation, the cable having acharacteristic impedance given by an empirical relation:

where e =the dielectric constant of the cable; D -=the horizontalspacing between conductors; D =the vertical spacing between cableconductor planes; al :the diameter of signal conductors and d =thediameter of ground conductors.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiment of the invention, as illustrated inthe accompanying drawings.

FIGURE 1 is an axial view of a tape cable disclosed in the prior art.

FIGURE 2 is an equipotential plot of the field intensity of oneconductor in the cable of FIGURE 1.

FIGURE 3 is an axial view of a flexible, multiple conductor transmissionline employing the principles of the present invention.

FIGURE 4 is an equipotential plot of the field intensity of oneconductor in the cable of FIGURE 3.

FIGURE 5 is an axial view of another cable employing the principles ofthe present invention.

Referring to FIGURE 1, a tape cable 2i presently available on thecommercial market includes an insulating member 22, typically vinyl orTeflon plastic, having embedded therein a plurality of conductors 24which serve as signal paths for individual signals received from signalsources 21, 23, 25 and 27 respectively. Secured to one side of the cableis a metallic member 26 which is connected to shield potential source 27and serves as a ground return conductor for the signal paths.Customarily, the tape cable 2i is referred to as a ground plane cable.For a signal conductor spacing of mils, a dielectric constant of 2, acharacteristic impedance of the order of 100 ohms, an attentuation of.15 db/lineal foot at 30 megacycles, a maximum DC. resistance of .25ohm/lineal foot for signal conductors, the thickness of the prior artcable is of the order of 30 to mils. For microminiaturized dataprocessing units, it has been determined that tape cables shouldwithstand a one time sharp creased fiat fold on themselves without anyfracture or decrease in insulation resistance. Laboratory experience hasindicated that metal-clad ground planes for the cable indicated in FIG-URE 1 will fracture when folded in the manner described. As a result,the attenuation of the cable is increased with the metallic shield ofsuch cables fractured. Crosstalk between the folded conductors alsoincreases to a prohibitive level.

Referring to FIGURE 2 an equipotential plot of the field intensity for aconductor 24 indicates the coupling between adjacent conductors. Theplot was made by constructing an analog model of the prior art cablescaled to the aforementioned dimensions and characteristic impedance inaccordance with the analog field plot technique described in ElectricalEngineering, September 1961, page 699. For purposes of the plot a 2 voltD.C. level was applied between the conductor 24 and the ground plane 26.A conventional vacuum tube voltmeter was employed to establish a .5 voltequipotential line 27. The voltage to ground at conductor 24' was .5volt and the ratio of the voltages to ground of conductors 24 and 24'was 1/4. The 1/4 ratio which represents the magnitude of the signallevel coupled from conductor 24 to conductor 24 will be employedhereinafter as a basis for comparison with the present invention.

FIGURE 3 discloses a tape cable 3% employing the principles of thepresent invention. The cable 31 which will be referred to hereinafter,is a barrier shielded cable. 1 The present invention, as will appearhereinafter, overcomes the limitations of prohibitive crosstalk andattenuation, previously enumerated, for the grounded plane cable 21?shown in FIGURE 1.

The cable 3% includes an insulating member 32, typical- 2Q ly Tefion orthe like, signal conductors 34, 3d, 3d of diameter d which are connectedrespectively to signal sources 51, 52, 53, 4, 55 and 56 and ground orshielding conductors 36, 36', 36 of diameter d which are connected toshield potential source 57 where 25 preferably but not exclusively d dIt should be noted that although six signal conductors are indicated,the cable is not necessarily limited to such a number but may having anyquantity depending upon the limitations of the cable fabricatingapparatus. The signal and ground or shielding conductors are arranged inthe insulating member 32 in conductor planes 33 and 35. Each conductorplane has alternate signal and ground conductors in juxtaposed relation.Successive conductor planes are arranged in offset relations, that, is aground conductor is asso- 35 ciated with each pair of superimposedconductors in the conductor planes. Thus, considering any set ofsuperimposed conductors in the conductor planes, it will be noted that aground conductor is included in each set of superimposed conductors.Additionally, adjacent sets of superimposed conductors are in invertedrelation. Thus, in one set of super-imposed conductors the signalconductor will be above the ground conductor whereas in the next oradjacent set of conductors, the ground conductor is above the signalconductors. The superimposed conductor sets are repeated in this manneralong the width of the conductor. The result of this conductorarrangement is to surround each signal conductor with at least threeground conductors. For example, the signal conductor 34' is surroundedby ground conductors 36, 36 and 3%". Signal conductors 34 are surroundedby ground conductors 36, 36" and 36". This grounding configurationprovides a shielding effect which reduces the crosstalk between adjacentdiagonally disposed signal conductors (34 and 34) as well as thecrosstalk between signal conductors (34 and 34') disposed in the sameplane.

For any characteristic impedance Z the particular dimensions of thesignal and ground conductor geometry necessary to obtain a tape cablewith reduced coupling is given by the following empirical relationdeveloped from the analog field plot for the cable of FIGURE 3:

Z =characteristic impedance of the line e =dielectric constant of theinsulation D horizontal spacing between conductors in a plane D=vertical spacing between conductor planes d =diameter of the signalconductors 7O d ==diameter of the ground conductors.

Referring to FIGURE 4, an equipotential plot of the field intensity fora conductor 34 of the cable shown in FIGURE 3 indicates the couplingbetween adjacent con- 7 ductors. The plot was constructed by the analogfield plot procedure previously described. The physical characteristicsof the line examined, that is, thickness, dielectric, conductor spacing,were the same as those for the grounded plane cable of FIGURE 1. Acomparison of the field plots for conductors in the cables of FIGURES 1and 3, as a result, indicates the relative electromagnetic radiation andline coupling of the lines. The radiation and coupling of a line, as iswell-known in the art, are proportional to the attenuation andcrosstalk, respectively, of the lines.

To facilitate comparison between the field plots, the .5 volt to groundequipotential line measurement is indicated in FIGURE 4-. It is apparentthat the .5 volt equipotential circle 41 of FIGURE 4 is of less diameterthan that for FIGURE 2. Hence, the crosstalk and attentuation of thecable of FIGURE 3 are less than that for FIGURE 1. Actually, the voltageto ground measured at conductor 34- was of the order of millivolts.Thus, the coupling or crosstalk ratio between diagonally disposed signalconductors 34, of FIGURE 3 is approximately 1/ 12.5 which is more than aone-third reduction in coupled voltage for the cable of FIGURE 3 thanthat of FIGURE 1.

The attenuation associated with the cable of FIGURE 3 is also reducedwith respect to that of FIGURE 1. Since attenuation is proportional tothe electromagnetic radiation extending from the cable, it is believedevident from FIGURES 2 and 4 that the radiation extends further fromFIGURE 1 than that for FIGURE 3. Hence, the attenuation of the latter isless than that for the former. Actual decibel measurements on the cablesindicated a .17 db/ft. loss for the cable of FIGURE 1 whereas the cableof FIGURE 3 had a .10 db/ft. loss.

Summarizing, Table I below provides comparative data or" cables ofsubstantially identical physical dimensions and electricalcharacteristics for the geometrical configurations indicated in FIGURES1 and 3:

Electrically, the barrier shielded cable has a crosstalk factorheretofore not possible in tape cables of conductor separations of theorder of tens of mils. The crosstalk factor is defined as the ratio indecibels of the signal level coupled into one conductor from an adjacentconductor. Employing the well-known relation V1 db-log V2 where V2 isthe voltage appearing on one conductor and V1 the coupled voltageappearing on an adjacent conductor, the crosstalk factor for the barriershielded cable is 22 db whereas that for the grounded plane conductor is--12 db. It is believed evident that a signal coupling of -12 db on atransmission line would sufficiently alter the signal on a line to apoint that it would not be useful in information processing. Thus, tapecables of the type shown in FIGURE 1 are not practicable inmicrominiaturized data processing apparatus. Laboratory experienceindicates, however, that crosstalk factors of 22 db are well within thesignal tolerances established for such apparatus.

Mechanically, the tape cable shown in FIGURE 3 has improved flexibiltydue to the ground plane being a series of individual conductors whichmay be readily folded without fracture of the conductors. Further, forgiven physical dimensions, the present invention has indicated thatimproved electrical characteristics are realized over 53 the prior artcable. Conversely, for identical electrical characteristics, the presentinvention provides reduced physical dimensions, i.e., thickness, spacingof conductors and wire size with respect to the prior art because of theimproved shielding and attenuation properties of the barrier shieldedcable. Tape cables employing the principles of present invention areobtainable with /3 to /2 thickness of the prior art cable.Correspondingly, tape cables of the present invention are obtainablewith /3 to /2 the Width of the prior art cables. The reduced thicknessand Width of the present invention provide improved fiexibilty forinterconnecting microminiaturized units.

Thus, in summary, the present invention has provided a new and improvedtape cable which has the required electrical and mechanicalcharacteristics necessary for interconnecting microminiaturized units ina data processing system.

Another embodiment of the present invention, shown in FIGURE 5, is atape cable 4%. Included in the cable 4%) is an insulating member 42,typically Teflon or the like, having signal conductors 44, 44', 447'which are connected respectively to signal sources 41, 43 and 4-5 andground conductors 46, 46', 46" which are connected to shield potentialsource 47, in juxtaposed relation in two or more superimposed conductorplanes. The cable conductors of FIGURE 5 are flat, however, as comparedto the cylindrical conductors of FIGURE 3. Actually, in bothembodiments, the conductors may be of any configuration. Alternateconductor planes are arranged in the manner described in connection withFIGURE 3, i.e., each set of superimposed conductors includes a groundconductor and a signal conductor. Alternate sets of superimposedconductors are in inverted order. The tape cable of FIGURE 5 includes anadditional feature which further improves the crosstalk factor andattenuation of the cable. Preparing equipotential plots of the fieldintensities of the cable conductors, in the manner previously described,produces coupling ratios of the order of 1/ 17. When converted, the l/17 coupling ratio corresponds to a crosstalk factor of -25 db withlittle or no sacrifice in attenuation. Thus, the cable of FIGURE 5 hasfurther improved pertormance over that of FIGURE 1.

The particular dimensions of the signal and ground conductor geometryfor the tape cable 44) is given by the following empirical relation(110%):

where signal and ground conductor thickness signal and ground conductorwidth; signal conductor thickness=ground conductor thickness; D-=center-to-center spacing of conductors in the same plane; D =spacingbetween horizontal axes for the adjacent conductor planes; e dielectricconstant and where IVg=th6 width of the signal conductors and T=thethickness of the signal conductor and W W Where W is the width of theground conductor.

The present invention depends, in large part, upon the geometricalrelation of the signal and ground conductors in an insulating member.The ground and signal conductors may be embedded in an insulating mediumor fabricated by well-known printed circuits or like techniques. Oneprocedure for fabricating the cable is to superimpose on a sheet ofTeflon or like plastic, a plurality of etched or wire conductors.Thereafter, a second sheet of plastic can be superimposed on the etchedor wire conductors followed by another plurality of conductors. A finalsheet or plastic can be superimposed on the top conductor and the entireassembly laminated together. The dimensions of the plastic sheets,conductors, dielectric constant should be selected in accordance withthe formulae pre viously indicated for the cable desired. Fabricatingsuch 6 conductors in a continuous process is well-known in the art asevidenced by US. Patent No. 2,849,298, issued on August 26, 1958.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A flexible, multiple, conductor transmission line comprising aninsulating member of suitable dielectric constant at least two conductorplanes included in the insulating member,

each conductor plane including a plurality of conductors of diameters dand d respectively,

alternate conductors in a plane being adapted as ground conductors,

the remaining conductors in a plane being adapted as signal conductors,the signal conductors having a diameter a and the ground conductorshaving a diameter d Where d d corresponding conductor positions in eachplane comprising a ground conductor and a signal conductor whereby eachsignal conductor is surrounded by at least three ground conductors and,for transmission lines having a characteristic impedance of the order ofohms, a spacing between the conductor planes and adjacent conductors ona conductor plane to provide a crosstalk ratio of 1/ 12.5 between asignal level coupled by the signal conductor to the next adjacent signalconductor and a signal level on the signal conductor.

2. A flexible, multiple, conductor transmission line comprising aninsulating member of suitable dielectric constant at least two conductorplanes included in the insulating member,

each conductor plane including a plurality of ground and signalconductors, alternate conductors being ground conductors, the

remaining conductors being signal conductors,

corresponding conductors in each conductor plane comprising a signalconductor and a ground conductor, and for transmission lines having acharacteristic impedance of the order of 100 ohms, a spacing betweenconductor planes and adjacent conductors in a plane given by:

100 3D,Dh

r (sewer/W2 where e =the dielectric constant of the insulating member; D=the horizontal spacing between the conductors in a conductor plane; D=the vertical spacing between conductor planes; d =the diameter ofsignal conductors; d =the diameter of ground conductors and Z =thecharacteristic of the transmission line.

3 A flexible, multiple, conductor transmission line comprising aninsulating member of suitable dielectric constant, a at least twoconductor planes included in the insulating member,

each conductor plane including a plurality of flat conductors injuxtaposed relation,

alternate flat conductors in each plane defined as ground conductors,

the remaining fiat conductors in the conductor planes defined as signalconductors,

corresponding conductor positions in each plane comprising a groundconductor and a signal conductor,

the ground conductor in each conductor position overlapping the groundconductor in the adjacent conductor position.

4. A flexible, multiple, conductor transmission line comprising:

an insulating member of suitable dielectric constant, at least twoconductor planes included in the insulating member, each conductor planeincluding a plurality of shielding and signal conductors, a plurality ofsignal sources connected to said signal conductors, and a shieldingpotential source connected to said shielding conductors, said signal andshielding conductors being arranged in the respective conductor planessuch that each conductor adjacent to a signal conductor in itsrespective conductor plane and in a direction normal to that plane is ashielding conductor. 5. A flexible, multiple, conductor transmissionline comprising:

an insulating member of suitable dielectric constant, at least twosuperimposed conductor planes included in the insulating member, and aplurality of conductors in juxtaposed relation included in eachconductor plane, alternate conductors in each plane being adapted asground conductors, the remaining conductors in each plane being adaptedas signal conductors, alternate conductor planes being horizontallyoffset with respect to its superimposed conductor plane wherebycorresponding conductors in each plane include a ground conductor and asignal conductor, a plurality of electric signal sources connected tosaid signal conductors, and electric potential sources connected to eachof said ground conductors. 6. A flexible, multiple, conductortransmission line comprising:

an insulating member having a lateral and longitudinal axis and being ofsuitable dielectric constant, a plurality of signal conductors includedin the member and extending parallel to said longitudinal axis, and alike plurality of ground conductors included in the member and extendingparallel to said longitudinal axis, selected ground and signalconductors comprising pairs of conductors with alternate pairs havingthe ground conductor normally superimposed above the signal conductor,said pairs being spaced along the lateral axis of the member,

the remaining pairs of conductors spaced along the lateral axis havingthe signal conductor superimposed above the ground conductor,

a plurality of electric signal sources connected to said signalconductors, and

10 electric potential sources connected to each of said groundconductors.

7. A flexible, multiple, conductor transmission line comprising:

an insulating member having a lateral axis and a longitudinal axis andbeing of suitable dielectric constant,

and

at least two conductor planes included in the insulating member inparallel relation with respect to the longitudinal axis of the member,

each conductor plane including a plurality of ground and signalconductors in spaced relation along the lateral axis of the member,

alternate conductors in each plane being adapted as ground conductors,

the remaining conductors in the conductor planes being adapted as signalconductors,

corresponding conductor positions in each conductor plane comprising asignal conductor and a ground conductor whereby each signal conductor issurrounded by a plurality of ground conductors,

a plurality of electric signal sources connected to said signalconductors, and

electric potential sources connected to each of said ground conductors.

References Cited by the Examiner UNITED STATES PATENTS 1,855,303 4/32McCurdy 3331 3,088,995 5/63 Baldwin 174-36 3,097,036 7/63 Cornelll74-l17 FORETGN PATENTS 1,124,245 6/56 France.

HERMAN KARL SAALBACH, Primary Examiner.

4. A FLEXIBLE, MULTIPLE, CONDUCTOR TRANSMISSION LINE COMPRISING: ANINSULATING MEMBER OF SUITABLE DIELECTRIC CONSTANT, AT LEAST TWOCONDUCTOR PLANES INCLUDED IN THE INSULATING MEMBER, EACH CONDUCTOR PLANEINCLUDING A PLURALITY OF SHIELDING AND SIGNAL CONDUCTORS, A SHIELDINGPOTENTIAL SOURCE CONNECTED TO SAID SHIELDING CONDUCTORS, AND A SHIELDINGPOTENTIAL SOURCE CONNECTED TO SAID SHIELDING CONDUCTORS, SAID SIGNAL ANDSHIELDING CONDUCTORS BEING ARRANGED IN THE RESPECTIVE CONDUCTOR PLANESSUCH THAT EACH CONDUCTOR ADJACENT TO A SIGNAL CONDUCTOR IN ITSRESPECTIVE CONDUCTOR PLANE AND IN A DIRECTION NORMAL TO THAT PLANE IS ASHIELDING CONDUCTOR.